Two model developmental pathways are being studied: 1) The conidiation pathway of Aspergillus nidulans,and 2) The head-assembly pathway of bacteriophage T4. The formation of asexual spores (conidia) by Aspergillus nidulans is a developmental process which can be induced only after a spore-originated colony has reached a certain age (the time of competence). Once competence is acquired, an inducing stimulus causes an abrupt shift in the mode of growth and, after a fixed maturation period, the appearance of new, mature conidia. The objectives of this aspect of the proposal are to study two main questions: 1) What is the nature of the change of state that renders a colony competent for induction of conidiation? 2) What is the nature of the control elements and the structure of the control network for the processes that occur following induction? The general approach is by mutational dissection of the conidiation pathway using thermo-sensitive mutants blocked in conidiation, mutants which express conidial functions constitutively, mutants which are precocious in the acquisition of competence, and mutants altered in membrane transport functions, which, our studies suggest, may be intimately connected to the developmental clock. We will continue to explore the structure, genetic determinants, and fate of the head assembly core of bacteriophage T4. A recently-discovered core protein (pip) will be studied in order to determine its function in head assembly and to deduce the mechanisms by which extensive changes in amino acid composition of a pip cleavage fragment (internal peptide II) arise during speciation.